Flow microfluorometry, supported by multiparameter data analysis is being applied to problems in multiple areas: 1) regulation of membrane potential by lymphocytes and neutrophils, 2) lymphocyte ontogeny and activation, 3) abnormalities in lymphocyte subpopulations associated with murine autoimmune disease, and 4) lymphocyte and monocyte analysis in human immunologic diseases, including systemic lupus erythematosus and acquired immunodeficiency syndrome (AIDS).